Multielectrode arrays (MEAs) have the potential to bridge the gap between in vivo and in-vitro studies of neuronal networks, However, the engineering required to use MEAs is a burden for most neurophysiology labs. In addition, the biophysical principles underlying the use of MEAs have not been rigorously verified, In this project, Tanner Research-is developing a new, easy-to-use, modular system comprising an electrode array laboratory instrument hardware, and custom software. Our MEA system will allow both recording and real-time feedback stimulation. In an experimental and theoretical effort at Caltech, we are quantitatively charac terizing the biophysics of recording and stimulation with MEAs. Real- time feedback stimulation will be automatically triggered by complex neural activity patterns specified by the researcher. We will sell NEA systems and provide custom electrode-array design and fabrication services to biology labs. Our product will provide more electrodes and greater stimulation capability than a competing system, at a lower price. Furthermore, since it is modular, neurophysiology labs will be able to begin using MEAs with an even smaller budget, and then expand to 128- channel electrode arrays and real-time feedback. In short, this project will improve the understanding of MEA biophysics and make MEAs widely available with enhanced functionality. PROPOSED COMMERCIAL APPLICATION: Commercial applications for the proposed MEA system include primary neurobiological research at some 500 laboratories nationwide. Additionally, the MEA system has broad applications as a diagnostic tool for studying the affects of drugs and toxins, add has potential for inexpensive and effective drug screening without the use of living animals.